Fix divide by zero in benchmark_speed_read_signals
Fix divide by zero in benchmark_speed_read_signals

If read times are fast enough (especially in Python 2, perhaps because of a lower precision performance counter?), division by zero can happen when calculating speed in `benchmark_speed_read_signals `. This commit catches this problem.
rgerkin/python-neo,NeuralEnsemble/python-neo,samuelgarcia/python-neo,INM-6/python-neo,apdavison/python-neo,JuliaSprenger/python-neo